Metformin enhances clearance of chylomicrons and chylomicron remnants in nondiabetic mildly overweight glucose-intolerant subjects

Metabolic Action of Metformin

Metformin, a cheap and safe biguanide derivative, due to its ability to influence metabolism, is widely used as a first-line drug for type 2 diabetes (T2DM) treatment. Therefore, the aim of this review was to present the updated biochemical and molecular effects exerted by the drug. It has been well explored that metformin suppresses hepatic glucose production in both AMPK-independent and AMPK-dependent manners. Substantial scientific evidence also revealed that its action is related to decreased secretion of lipids from intestinal epithelial cells, as well as strengthened oxidation of fatty acids in adipose tissue and muscles. It was recognized that metformin’s supra-therapeutic doses suppress mitochondrial respiration in intestinal epithelial cells, whereas its therapeutic doses elevate cellular respiration in the liver. The drug is also suggested to improve systemic insulin sensitivity as a result of alteration in gut microbiota composition, maintenance of intestinal barrier integrity, and alleviation of low-grade inflammation.

Metformin and Systemic Metabolism

Metformin can improve patients' hyperglycemia through significant suppression of hepatic glucose production. However, up to 300 times higher concentrations of metformin accumulate in the intestine than in the circulation, where it alters nutrient metabolism in intestinal epithelial cells and microbiome, leading to increased lactate production. Hepatocytes use lactate to make glucose at the cost of energy expenditure, creating a futile intestine-liver cycle. Furthermore, metformin reduces blood lipopolysaccharides and its initiated low-grade inflammation and increased oxidative phosphorylation in liver and adipose tissues. These metformin effects result in the improvement of insulin sensitivity and glucose utilization in extrahepatic tissues. In this review, I discuss the current understanding of the impact of metformin on systemic metabolism and its molecular mechanisms of action in various tissues.

Impact of glucose-lowering drugs on cardiovascular disease in type 2 diabetes

Type 2 diabetes mellitus (T2DM) is characterized by multiple pathophysiologic abnormalities. With time, multiple glucose-lowering medications are commonly required to reduce and maintain plasma glucose concentrations within the normal range. Type 2 diabetes mellitus individuals also are at a very high risk for microvascular complications and the incidence of heart attack and stroke is increased two- to three-fold compared with non-diabetic individuals. Therefore, when selecting medications to normalize glucose levels in T2DM patients, it is important that the agent not aggravate, and ideally even improve, cardiovascular risk factors (CVRFs) and reduce cardiovascular morbidity and mortality. In this review, we examine the effect of oral (metformin, sulfonylureas, meglitinides, thiazolidinediones, DPP4 inhibitors, SGLT2 inhibitors, and α-glucosidase inhibitors) and injectable (glucagon-like peptide-1 receptor agonists and insulin) glucose-lowering drugs on established CVRFs and long-term studies of cardiovascular outcomes. Firm evidence that in T2DM cardiovascular disease can be reversed or prevented by improving glycaemic control is still incomplete and must await large, long-term clinical trials in patients at low risk using modern treatment strategies, i.e., drug combinations designed to maximize HbA1c reduction while minimizing hypoglycaemia and excessive weight gain.

New Insights into the Regulation of Chylomicron Production

Dietary lipids are efficiently absorbed by the small intestine, incorporated into triglyceride-rich lipoproteins (chylomicrons), and transported in the circulation to various tissues. Intestinal lipid absorption and mobilization and chylomicron synthesis and secretion are highly regulated processes. Elevated chylomicron production rate contributes to the dyslipidemia seen in common metabolic disorders such as insulin-resistant states and type 2 diabetes and likely increases the risk for atherosclerosis seen in these conditions. An in-depth understanding of the regulation of chylomicron production may provide leads for the development of drugs that could be of therapeutic utility in the prevention of dyslipidemia and atherosclerosis. Chylomicron secretion is subject to regulation by various factors, including diet, body weight, genetic variants, hormones, nutraceuticals, medications, and emerging interventions such as bariatric surgical procedures. In this review we discuss the regulation of chylomicron production, mechanisms that underlie chylomicron dysregulation, and potential avenues for future research.

Does long-term metformin treatment increase cardiac lipoprotein lipase?

Acute activation of adenosine monophosphate-activated protein kinase (AMPK) or jumps in cardiac work increased cardiac endothelial lipoprotein lipase (LPL), yet it is unclear whether chronic AMPK activation maintains this elevated LPL. To activate AMPK chronically, metformin at low (300 mg/kg/d) and high dose (600 mg/kg/d) was administered in drinking water for 14 days. Control, metformin-treated, and 5-amino-imidazole-4-carboxamide riboside (AICAR)-treated (0.5 mmol/L) ex vivo hearts were perfused to investigate uptake of triacylglycerol and cardiac LPL activity. For perfused rat hearts, increased uptake of labeled Intralipid and β-oxidation of Intralipid-fatty acid were noted for both AICAR (P < .05) and high-dose metformin (P < .01). Intralipid incorporation into tissue lipids was decreased by AICAR (P < .05) and increased after high-dose metformin (P < .05), the increase manifest as enhanced triacylglycerol deposition (P < .05). Low-dose metformin did not alter lipid uptake or tissue deposition. Both high-dose metformin and AICAR decreased cardiac acetyl-coenzyme A carboxylase activity (P < .01). Heparin-releasable LPL was increased after treatment with AICAR (P < .05) and high-dose metformin (P < .01). Low-dose metformin did not alter cardiac LPL. High-dose metformin doubled immunoreactive AMPK and phospho-AMPK protein (P < .001) and increased phosphorylation of p38-mitogen-activated protein kinase (P < .05). After heparin pretreatment, the rate of recruitment of LPL to the cardiac endothelium was increased by AICAR (P < .05) but not by high-dose metformin. These data suggest that AMPK activation increased cardiac endothelial LPL, yet acute and chronic activation of AMPK may yield increased LPL through differing mechanisms.

Effects of pioglitazone and metformin on NEFA-induced insulin resistance in type 2 diabetes

AIMS/HYPOTHESIS

We sought to determine whether pioglitazone and metformin alter NEFA-induced insulin resistance in type 2 diabetes and, if so, the mechanism whereby this is effected.

METHODS

Euglycaemic-hyperinsulinaemic clamps (glucose approximately 5.3 mmol/l, insulin approximately 200 pmol/l) were performed in the presence of Intralipid-heparin (IL/H) or glycerol before and after 4 months of treatment with pioglitazone (n = 11) or metformin (n = 9) in diabetic participants. Hormone secretion was inhibited with somatostatin in all participants.

RESULTS

Pioglitazone increased insulin-stimulated glucose disappearance (p < 0.01) and increased insulin-induced suppression of glucose production (p < 0.01), gluconeogenesis (p < 0.05) and glycogenolysis (p < 0.05) during IL/H. However, glucose disappearance remained lower (p < 0.05) whereas glucose production (p < 0.01), gluconeogenesis (p < 0.05) and glycogenolysis (p < 0.05) were higher on the IL/H study day than on the glycerol study day, indicating persistence of NEFA-induced insulin resistance. Metformin increased (p < 0.001) glucose disappearance during IL/H to rates present during glycerol treatment, indicating protection against NEFA-induced insulin resistance in extrahepatic tissues. However, glucose production and gluconeogenesis (but not glycogenolysis) were higher (p < 0.01) during IL/H than during glycerol treatment with metformin, indicating persistence of NEFA-induced hepatic insulin resistance.

CONCLUSIONS/INTERPRETATION

We conclude that pioglitazone improves both the hepatic and the extrahepatic action of insulin but does not prevent NEFA-induced insulin resistance. In contrast, whereas metformin prevents NEFA-induced extrahepatic insulin resistance, it does not protect against NEFA-induced hepatic insulin resistance.

Novel aspects of postprandial lipemia in relation to atherosclerosis

Postprandial hyperlipidemia is considered to be a substantial risk factor for atherosclerosis. Interestingly, this concept has never been supported by randomized clinical trials. The difficulty lies in the fact that most interventions aimed to reduce postprandial lipemia, will also affect LDL-C levels. The atherogenic mechanisms of postprandial lipids and lipoproteins can be divided into direct lipoprotein-mediated and indirect effects; the latter, in part, by inducing an inflammatory state. Elevations in postprandial triglycerides (TG) have been related to the increased expression of postprandial leukocyte activation markers, up-regulation of pro-inflammatory genes in endothelial cells and involvement of the complement system. This set of events is part of the postprandial inflammatory response, which is one of the recently identified potential pro-atherogenic mechanisms of postprandial lipemia. Especially, complement component 3 levels show a close correlation with postprandial lipemia and are also important determinants of the metabolic syndrome. In clinical practice, fasting TG are frequently used as reflections of postprandial lipemia due to the close correlation between the two. The use of serial capillary measurements in an out-of-hospital situation is an alternative for oral fat loading tests. Daylong TG profiles reflect postprandial lipemia and are increased in conditions like the metabolic syndrome, type 2 diabetes and atherosclerosis. Studies are needed to elucidate the role of postprandial inflammation in atherogenesis and to find new methods in order to reduce selectively the postprandial inflammatory response. Future studies are needed to find new methods in order to reduce selectively the postprandial inflammatory response.

Impact of metformin versus the prandial insulin secretagogue, repaglinide, on fasting and postprandial glucose and lipid responses in non-obese patients with type 2 diabetes

OBJECTIVE

Non-obese patients with type 2 diabetes (T2DM) are characterized by predominant defective insulin secretion. However, in non-obese T2DM patients, metformin, targeting insulin resistance, is non-inferior to the prandial insulin secretagogue, repaglinide, controlling overall glycaemia (HbA1c). Whether the same apply for postprandial glucose and lipid metabolism is unknown. Here, we compared the effect of metformin versus repaglinide on postprandial metabolism in non-obese T2DM patients.

DESIGN

Single-centre, double-masked, double-dummy, crossover study during 2x4 months involving 96 non-obese (body mass index < or = 27 kg/m2) insulin-naïve T2DM patients. At enrolment, patients stopped prior oral hypoglycaemic agents therapies and after a 1-month run-in period on diet-only treatment, patients were randomized to repaglinide (2 mg) thrice daily followed by metformin (1 g) twice daily or vice versa each during 4 months with 1-month washout between interventions.

METHODS

Postprandial metabolism was evaluated by a standard test meal (3515 kJ; 54% fat, 13% protein and 33% carbohydrate) with blood sampling 0-6 h postprandially.

RESULTS

Fasting levels and total area under the curve (AUC) for plasma glucose, triglycerides and free fatty acids (FFA) changed equally between treatments. In contrast, fasting levels and AUC of total cholesterol, low-density lipoprotein (LDL) cholesterol, non-high-density lipoprotein (non-HDL) cholesterol and serum insulin were lower during metformin than repaglinide (mean (95% confidence intervals), LDL cholesterol difference metformin versus repaglinide: AUC: -0.17 mmol/l (-0.26; -0.08)). AUC differences remained significant after adjusting for fasting levels.

CONCLUSIONS

In non-obese T2DM patients, metformin reduced postprandial levels of glycaemia, triglycerides and FFA similarly compared to the prandial insulin secretagogue, repaglinide. Furthermore, metformin reduced fasting and postprandial cholesterolaemia and insulinaemia compared with repaglinide. These data support prescription of metformin as the preferred drug in non-obese patients with T2DM targeting fasting and postprandial glucose and lipid metabolism.

Pathophysiology and treatment of the dyslipidemia of insulin resistance

Insulin resistance, and the compensatory hyperinsulinemia that results, has been linked to a host of defects including glucose intolerance, diabetes, hypertension, dyslipidemia, endothelial dysfunction, impaired fibrinolysis, and subclinical inflammation. Patients with this metabolic syndrome have a markedly increased risk for the development of atherothrombotic cardiovascular disease. The characteristic dyslipidemia of insulin resistance consists of elevated triglyceride and triglyceride-rich lipoprotein levels, low levels of high-density lipoprotein cholesterol, and increased concentrations of small, dense low-density lipoprotein cholesterol. Management of this dyslipidemia typically involves a dual approach. Lifestyle modification is an essential component of any successful treatment plan, but alone is usually insufficient to correct these lipoprotein abnormalities. Medications that diminish insulin resistance and directly alter lipoproteins are also necessary in the majority of cases. Combinations of therapeutic agents are often required to optimize attainment of treatment goals.

Metabolic effects of metformin in patients with impaired glucose tolerance

AIMS

To assess the effect of metformin on insulin sensitivity, glucose tolerance and components of the metabolic syndrome in patients with impaired glucose tolerance (IGT).

METHODS

Forty first-degree relatives of patients with Type 2 diabetes fulfilling WHO criteria for IGT and participating in the Botnia study in Finland were randomized to treatment with either metformin 500 mg b.i.d. or placebo for 6 months. An oral glucose tolerance test (OGTT) and a euglycaemic hyperinsulinaemic clamp in combination with indirect calorimetry was performed at 0 and 6 months. The patients were followed after stopping treatment for another 6 months in an open trial and a repeat OGTT was performed at 12 months.

RESULTS

Metformin treatment resulted in a 20% improvement in insulin-stimulated glucose metabolism (from 28.7 +/- 13 to 34.4 +/- 10.7 micromol/kg fat-free mass (FFM)/min) compared with placebo (P = 0.01), which was primarily due to an increase in glucose oxidation (from 16.6 +/- 3.6 to 19.1 +/- 4.4 micromol/kg FFM; P = 0.03) These changes were associated with a minimal improvement in glucose tolerance, which was maintained after 12 months.

CONCLUSIONS

Metformin improves insulin sensitivity in subjects with IGT primarily by reversal of the glucose fatty acid cycle. Obviously large multicentre studies are needed to establish whether these effects are sufficient to prevent progression to manifest Type 2 diabetes and associated cardiovascular morbidity and mortality. Diabet. Med. 18, 578-583 (2001)

Improved metabolic control reduces the number of postprandial apolipoprotein B-48-containing particles in type 2 diabetes

Postprandial lipoproteins are raised in diabetes and there is increasing evidence for the atherogenicity of the chylomicron remnant. Increased postprandial cholesteryl ester transfer has also been demonstrated in diabetes and may contribute to the atherogenic lipoprotein profile. The present study examined the effect of improving metabolic control on postprandial lipoproteins in 13 Type 2 diabetic patients. Blood was taken fasting and at 2-h intervals following a high fat, 1100 kcal meal. Patients were brought into good control by intensified dietary advice and oral hyperglycaemic agents or insulin if blood glucose failed to respond. Fasting and postprandial cholesteryl ester transfer protein (CETP) and lecithin:cholesteryl acyltransferase (LCAT) were determined in six patients. Lipoproteins were isolated by sequential ultracentrifugation. Chylomicron and very low density lipoprotein (VLDL) apolipoprotein B-48 and apolipoprotein B-100 were isolated by polyacrylamide gradient gel electrophoresis and quantified by densitometric scanning. CETP and LCAT were determined by an endogenous method which determined cholesterol esterification and transfer between the patients' lipoproteins. There was a significant reduction in postprandial chylomicron apo B-48 (P<0.005), apo B-100 (P<0.0005) and chylomicron cholesterol (P<0.001) following improved diabetic control. The chylomicron lipid/apo B ratio increased with improved control (P<0.01). Postprandial CETP and LCAT were significantly reduced in good control (P<0.01 and P<0.05, respectively) and there were significant changes in HDL composition. The study shows that improvement in metabolic control in Type 2 diabetic patients leads to a reduction in postprandial chylomicron particles and less transfer of cholesterol to apo B-containing lipoproteins.

Treatment with metformin of non-diabetic men with hypertension, hypertriglyceridaemia and central fat distribution: the BIGPRO 1.2 trial

BACKGROUND

In the BIGPRO 1 trial, one year of treatment with metformin in non-diabetic obese subjects with a central fat distribution had no significant effect on fasting plasma triglyceride concentration or on blood pressure despite a decrease in weight, fasting plasma insulin and glucose concentrations. To re-evaluate the effect of metformin on fasting triglyceride concentration and on blood pressure, the BIGPRO 1.2 trial included non-diabetic men (n=168) with a fasting plasma triglyceride concentration > or =1.7 and < or =6.5 mmol/l, high blood pressure (systolic > or =140 and < or =180 and/or diastolic > or =90 and < or =105 mmHg, or treatment for hypertension) and a waist-to-hip ratio > or =0.95.

METHODS

A randomised double-blind trial comparing metformin treatment (850 mg bid) with placebo.

RESULTS

Metformin had no significant effect either on blood pressure or plasma triglyceride concentration. In comparison with the placebo group, fasting plasma insulin (p<0.04), total cholesterol (p<0.05) and Apo B (p<0.008) concentrations decreased more in the metformin group in the BIGPRO 1. 2 trial, confirming most of the previous results of the BIGPRO 1 trial. Tissue plasminogen activator antigen concentration decreased significantly (p<0.01) only in the metformin group, but this was not significantly different from the placebo group (p<0.12); further, there were no significant differences in the change in plasminogen activator inhibitor 1.

CONCLUSIONS

The consistency of the two BIGPRO trials supports the conclusion that metformin affects several cardiovascular risk factors favourably in non-diabetic subjects with a central fat distribution.

A risk-benefit assessment of metformin in type 2 diabetes mellitus

Metformin has been used for over 40 years as an effective glucose-lowering agent in type 2 (noninsulin-dependent) diabetes mellitus. Typically it reduces basal and postprandial hyperglycaemia by about 25% in more than 90% of patients when either given alone or coadministered with other therapies including insulin during a programme of managed care. Metformin counters insulin resistance and offers benefits against many features of the insulin resistance syndrome (Syndrome X) by preventing bodyweight gain, reducing hyperinsulinaemia and improving the lipid profile. In contrast to sulphonylureas, metformin does not increase insulin secretion or cause serious hypoglycaemia. Treatment of type 2 diabetes mellitus with metformin from diagnosis also offers greater protection against the chronic vascular complications of type 2 diabetes mellitus. The most serious complication associated with metformin is lactic acidosis which has an incidence of about 0.03 cases per 1000 patients years of treatment and a mortality risk of about 0.015 per 1000 patient-years. Most cases occur in patients who are wrongly prescribed the drug, particularly patients with impaired renal function (e.g. serum creatinine level > 130 micromol/L or > 1.5 g/L). Other major contraindications include congestive heart failure, hypoxic states and advanced liver disease. Serious adverse events with metformin are predictable rather than spontaneous and are potentially preventable if the prescribing guidelines are respected. Gastrointestinal adverse effects, notably diarrhoea, occur in less than 20% of patients and remit when the dosage is reduced. The life-threatening risks associated with metformin are rare and could mostly be avoided by strict adherence to the prescribing guidelines. Given the 4 decades of clinical experience with metformin, its antihyperglycaemic efficacy and benefits against Syndrome X, metformin offers a very favourable risk-benefit assessment when compared with the chronic morbidity and premature mortality among patients with type 2 diabetes mellitus.

Postprandial lipid metabolism in diabetes

Postprandial lipemia is an inherent feature of diabetic dyslipidemia and highly prevalent in diabetic patients even with normal fasting triglyceride concentrations. Postprandial lipemia is characterized by long residence time of chylomicron and VLDL remnants in the circulation. Insulin resistance causes increased flux of free fatty acids, and thus enhanced VLDL apolipoprotein B (apo B) synthesis in the liver. Together with chylomicron and VLDL remnant competition for the common removal mechanisms the increased substrate input results in exaggerated and prolonged postprandial lipemia. Studies using both apo B-48 and retinyl esters as a marker for intestinally derived particles have shown that increased postprandial lipemia does not predict the presence or absence of coronary artery disease between non-insulin-dependent diabetes mellitus (NIDDM) subjects. Recent data have shown that postprandial triglyceride-rich remnants are atherogenic, and postprandial hypertriglyceridemia contributes to the metabolic disturbances transforming LDL and HDL subclasses into more atherogenic direction in diabetic subjects.